It is desirable in aircraft shock absorbers to be able to adjust the degree of throttling as a function of the conditions under which the landing gear containing the shock absorber is being used, and in particular the throttling that applies during expansion of the hydraulic fluid contained in the shock absorber. It is also known that when the characteristics of the shock absorber need to be changed while it is in use, the general practice to use an external source of fluid under pressure acting on a piston moving inside the shock absorber.